An optical printer is a device that can convert a digitally stored image into light, and can project this light onto a light-sensitive medium to generate an image. Optical printers were originally used to form photoresist masks for electronic circuit boards, and were relatively easy to manufacture as the optical exposure was either “on” or “off”.
The variant of optical printer described in this patent is predominantly designed for producing photographic-style images, although the teachings of the present invention are not limited to the production of photographic-style images.
Optical printers are distinct from laser-printers and inkjet printers as they do not deposit material (toner in the case of laser-printers and ink in the case of ink-jet printers) onto the medium, instead exposing the medium to light, which is subsequently chemically processed. Whilst the name “printer” is therefore something of a misnomer (in that such devices do not actually “print”, in the conventional sense of the word, anything onto the medium), devices of the type described herein are commonly known in the art as printers.
Photographic images are usually considered to be of a higher quality than ink- or laser-jet images, as photographic medium (e.g. photographic paper) has the capability to vary the intensity of each pixel in the image. Ink or laser-jet printers deposit material that is either there, or isn't. Consequently, the apparent resolution of an ink- or laser-jet printer (in dots-per-inch, or DPI) needs to be significantly higher than that of a photo-printer to achieve similar quality.
Several styles of optical photographic printer are commercially available, and use a variety of optical exposure mechanisms, including lasers, liquid-crystal masks, micro-mirrors and light-emitting diodes.
FIG. 1 is a schematic illustration of the typical components of an optical printer. As shown, the printer comprises a media canister from which a continuous length of unexposed photographic media (such as photographic paper) is drawn. The media is drawn from the canister by precision mechatronics, the media transporter, which provides media to the print head and positions the media with respect thereto. The print head comprises a plurality of optical elements (such as light emitting diodes) that can be energised to generate light which exposes the region of the media located immediately underneath the print head.
The print head is moved over the media to expose the media in swathes, and data is fed to the print head by a control system which arranges for the correct combination of elements to be energised to expose the media in such a way that the media is exposed with a desired final image.
Once the media has been exposed it is passed through the cutter by the media transporter and the cutter is operated to cut the exposed section of media from the remaining unexposed length of media. The exposed length of media is then passed to a chemical developer for developing. The developing process reveals the image on the media, and the finished printed image is output.
As shown, each of the elements of the system is carefully controlled by a control system. The control system may comprise a personal computer running appropriate software, purpose-built electronic circuits or a combination of the two.
For all printers, a key point of concern is the resolution at which the printer can print and much work has been done, particularly in the field of ink jet printing, to improve the number of dots per inch that can be printed on a substrate (typically paper). In an optical printer a key limiting factor for the achievable resolution is the size of the illumination device used to generate the light that impinges on the photographic medium.
Our previous patent (European Patent No. 1228633) discloses an illumination device (depicted in FIGS. 2(a) and 2(b) hereafter) that comprises three arrays of LED (light emitting diode) illumination elements 14—one column of red LEDs 20, one column of green LEDs 21 and one column of blue LEDs 22—mounted on a substrate 15. The illumination device is arranged to emit light, when energised, towards a light receiving face 13 of a tapered optic fibre light pipe 10. The light pipe conveys the light received from the LEDs at the light receiving face to a light emitting face (not shown) which has a smaller surface area that the light receiving face. Light emitted by the light emitting face then impinges on a photographic medium (such as photographic paper) to generate an image. As depicted in FIG. 2(a) the LEDs are mounted on the substrate in parallel columns, and (as shown in FIG. 2(b)) the LEDs of each respective column are spaced from adjacent LEDs to enable the necessary electrical connections to be made.
Tapering the optic fibre light pipe in the manner described in this patent reduces the size of the illuminated “spot” generated by the illumination device on the photographic medium (as compared to that which would otherwise be produced were the illumination device to directly illuminate the photographic medium), thereby enabling the optical resolution of the resulting photographic image to be increased.
Whilst this arrangement has greatly increased the resolution that was previously achievable with previous optical printers, there is a desire in the market for higher resolution images and to respond to this market need we have attempted to further improve the achievable resolution by reducing yet further the physical size of the illumination device. However, we have found that our attempts to reduce the physical size of the illumination device have been hampered inter alia by the physical size of the—typically LED—light emitting elements of the red, green and blue arrays that make up the illumination device, and the spacing between the elements that must be provided to enable the elements to be connected to an electrical power source.
A first aspect of the present invention seeks to address this technical problem of how to reduce the size of the illumination device given the constraints of the minimum physical size of the light emitting elements, and the space that must be provided to enable those elements to be coupled to a source of power.
A second aspect of the present invention is concerned with addressing issues concerning the quality of the printed image, in particular the quality of the image as perceived by the human eye.
With previously proposed devices, such as that disclosed in our prior patent, we have noticed that the quality of the image can sometimes be adversely affected by a so-called “banding” effect that can occur at the junction between successive printing swathes as the image is being formed on the photographic medium.
As is illustrated schematically in FIG. 2(c), photographic medium 1 (typically comprising a roll of photographic paper) is transported through the printing apparatus in a direction A, and the illumination device 3 (consisting of red, green and blue light emitting element arrays 5) is moved from point B to point C to illuminate a swathe of the photographic medium immediately below the device and generate part of the final image. Light from the elements, as previously described, impinges on the light receiving face of a tapered light pipe provided between the illumination device and the photographic medium 1 (hence the light pipe is not visible in FIG. 2(c)), and exits from the light pipe via a light transmitting face that is considerably smaller than the light receiving face to thereby illuminate an area of the photographic medium that is much less than that which would otherwise be illuminated were the arrays to directly illuminate the photographic medium. The illumination device then returns from point C to point B, the photographic medium is advanced by a distance X that is substantially equal to the length of the aforementioned light emitting face of the tapered light pipe (and smaller than the length of the illumination device), and the illumination device is then moved from point B to point C to print to print the next swathe of the image.
Whilst this arrangement works well, apparently insignificant errors in the amount paper advanced between the printing of successive swathes, and other operational irregularities, can detrimentally affect the final image. For example, advancing the paper by a length greater than X can cause a detectable gap (visible as a white line) between successive swathes, whereas advancing the paper by a length less than X can result in an overexposure of the photographic medium (visible as a noticeably darker band) in the region where the swathes overlap.
The aforementioned second aspect of the present invention seeks to address this technical problem of how to improve the quality of the printed image, in particular the quality of the image as perceived by the human eye (particularly, but not exclusively, to reduce the impact of the above described “banding” effect).
A third aspect of the present invention is concerned with the structure of the illumination device, and the printing head incorporating that device.
Our previously proposed illumination device is essentially as depicted in FIG. 2(b), and comprises a substrate 15 on which the individual light emitting elements 14 are mounted. The substrate is then encapsulated within a transparent layer 33 to protect the LEDs and the electrical connections 34 thereto. The illumination device is spaced from the tapered optic fibre light pipe 10, and an anti reflection layer 32 may be added to the transparent layer or to the light receiving face 13 of the light pipe 10.
Whilst this previously proposed illumination device has functioned adequately, we have noted that the quality of the final image can sometimes be adversely affected by crosstalk between adjacent LED elements, and by variations in the relative intensities of the red, green and blue arrays. It is also the case that maintenance of the gap between the illumination device and fibre optic light pipe must be carefully controlled (to avoid aberrations in the final image) and such control can be difficult to achieve in practice.
The third aspect of the present invention seeks to address such problems.
A fourth aspect of the present invention is concerned with calibration of the optical printer so as to improve the quality of the final image.
A persistent problem for optical printers, particularly those printers that use LEDs for light sources, is that the characteristics of the light sources vary dramatically from one to the other. For example, for any two LEDs of a particular colour both the intensity profile and wavelength profile of the light emitted by the LEDs, when driven by the same current, will vary considerably. These variations from one LED to another can show up as lines within swathes of a final printed image. It is also the case that light emitting diodes tend to vary non-linearly in both their output intensity and wavelength with input voltage variations, and a compounding problem is that the photographic medium (typically photographic paper) used for a particular print will directly affect the quality of that print and, furthermore, the characteristics of a given photographic paper will typically be quite different from those of another type of paper.
All of these different variations must be addressed if the quality of the final image is to be improved, and to this end it has previously been postulated that one could employ a series of photocells as a photographic medium, and feed back the output of those photocells to the light source elements to adjust the output of those elements and hence provide a uniform illumination of the paper.
However, whilst this would appear at first sight to be an attractive means of mitigating such problems, it is unfortunately the case that photocells respond very differently to photographic medium, such as photographic paper, and as such adjusting the system to be best suited for a set of photocells will not necessarily provide an appropriate setting for printing on photographic paper.
The fourth aspect of the present invention addresses these problems, and in general terms relates to the concept of calibrating a printer from one or more processed images printed with that printer. This approach is advantageous as the image will of course incorporate any of the aforementioned variations, and as such by calibrating to the image one inevitably addresses any such variations.
A fifth aspect of the present invention is concerned with improving the transport mechanism for the photographic medium.
A particular requirement of that transport mechanism is that to enable the printer to achieve a high resolution and to provide a high quality output, the components of the transport mechanism must be carefully designed and controlled to enable photographic medium movement with a tolerance of in the region of a few microns, typically roughly four microns. By this we mean that when it is desired to move the photographic medium a given distance through the printer, then the mechatronics of the transport mechanism must be such that the actual transport distance is within plus or minus up to a few microns (e.g. four microns) of the desired distance.
As will no doubt be appreciated by persons skilled in the art, if the actual transport distance should be greater than the desired distance then visible white bands may be produced in the final image. Similarly, if the transport distance should be less than the desired distance by more than four microns, then adjacent swathes of the image will overlap to a noticeable degree and the final image will be spoiled. It is critical, therefore, that the transport mechanism design is such that highly accurate photographic medium transport is enabled, and a fifth aspect of the present invention addresses this problem.
A sixth aspect of the present invention also relates to the transport of photographic medium through an optical printer. This aspect of the present invention relates to the fact that we have identified that it would be advantageous if a printer could be devised that provided a greater throughput of photographic medium. It would also be advantageous if such a printer could readily accommodate photographic medium of different sizes.
One relatively simple way to increase photographic medium throughput would be to increase the speed at which the optical head assembly travels over the photographic medium, however such an approach would not provide that great an increase in speed as the head must be over a given point on the photographic medium for a fixed minimum amount of time (for a given photographic medium and a given light output from the light source) to expose that point to the requisite amount of optical energy required to generate the image, and hence it is impossible—without adversely affecting the quality of the final image—to increase the speed of the head to a point where it is over a given point of the photographic medium for less than this fixed minimum period of time.
The sixth aspect of the present invention addresses these problems.